Bellatrix got up, trying to shake off the memories and focus on her present dilemma. How would she get out of here? She could, of course, go back to the lake, the detection charms would have probably worn off by now, but a bubblehead-charm would exhaust her further and she probably wouldn't be able to defend herself afterwards, at least not until she had found something to eat. Aaand she probably wouldn't have found the way back anyways.
So, what then? How could she find another way out? There was one way... she hadn't tried it in years, and it would probably drain a little more than just her magic from her, but it was the only thing she could think of. Raising her wand she concentrated strongly, trying, hard as she could, to remember her best memory. And she recalled... nothing. No silver shimmer of hope was to be found. Her mind skipped through years trying to find one single moment when she had been happy, truly happy. She landed on... something. She could feel it there, a memory shimmering in the same light the charm would glow, but she could not quite grasp it. Concentrating as hard as she could, Bella tried to remember. They had been very young when it happened. She about 9 maybe. Cissa had been only 5 then, and Andromeda about 7. They had all been in the garden together, along with... who? A nanny, a young girl of 16 who, if she remembered right, had been named July. She had told them things that would have gotten her killed had their mother heard of it. About muggles and muggle-borns, and about Ravenclaws and Hufflepuffs. About heroes who won a battle due to clever tricks rather than brute force, and about kings and queens who avoided war by managing to ally with other countries.
Yes! That was her memory. It shone a bright, clear silver in her mind, reminding her of moonlight. She tried with all her will to keep that memory, and only that one memory in her mind as her fingers performed the precise movements needed to cast the spell, and she whispered the incantation: „Expecto Patronum" a bright silver crow burst out of the tip of her wand, flapping its wings and hovering in the air in front of her, looking expectant. „find me a way out of here" Bella said to her Patronus, looking at it in awe, as she hadn't really expected to still be able to cast that particular happy spell. The crow took off silently, leaving Bellatrix behind in the darkness, and in the blink of an eye it was gone.